Magnetic recording devices such as read or write heads as well as other micro devices are fabricated using photolithography. Typically, a photoresist layer is deposited. Selected areas of the photoresist layer are exposed to light. Corresponding portions of the photoresist layer are removed, forming a photoresist mask. One or more layers below apertures the photoresist mask may be removed or one or more layers deposited to form structures in the apertures. The photoresist mask is removed. This process is repeated to fabricate other layers in the photolithographically defined device. The device may thus be formed layer by layer.
Overlay refers to the match (or mismatch) between the desired locations of structures in different layers of the device. If a substrate were to be considered a planar surface described by x and y coordinates (z is perpendicular to the surface), then overlay error may be considered to be the actual x and y coordinates of a structure versus the designed x and y coordinates based on the previous layer(s). For example, the pole tip of a writer and a read sensor of a reader may be desired to be aligned in the cross-track direction. These structures are part of different layers formed at different times. Misalignments between the read sensor and pole tip may be due to overlay errors.
Overlay errors may be due to a variety of factors. For example, optical distortions may occur in the apparatus used to expose the photoresist to light. These distortions can cause offsets in the locations of regions exposed to light. The structures being formed may, therefore, be offset from their desired location and, therefore, be subject to overlay error. Another source of overlay error is hot spots. Hot spots occur because particulate contamination on the back side of the substrate. These micrometer to nanometer sized particles become trapped between the substrate and the holder, causing the substrate to deform. This deformation may result in a distortion of the features in the image being transferred to the substrate. Thus, significant overlay errors in the local area of the particle. The region in which such errors occur is termed a hot spot.
FIG. 1 depicts a conventional method for interrogating overlay errors that may be due at least in part to the presence of hot spots. The overlay data has been measured for one or more layers on the substrate. Based on this data, it is determined whether the overlay error (as measured by a) exceeds a particular threshold, via step 12. If so, then the wafer is further inspected, via step 14. In some cases, the inspection takes the form of a highly trained operator inspecting the overlay vector map. The overlay vector map indicates the magnitude and direction of distortions in the overlay. The operator may thus manually determine whether hotspots are present based on the character of the overlay vector map. Alternatively, the flatness of the substrate may be measured in a tool. These procedures may determine that there are small regions in which the overlay error is significant. These localized regions of high overlay error may correspond to the locations of hotspots. Thus, the presence and location of hotspots may be determined. A determination may then be made as to whether to pass the substrate for further processing, discard the substrate or rework the substrate due to the overlay error.
Although the conventional method 10 provides a mechanism for detecting and addressing overlay error, it is time consuming, less sensitive to hot spots than desired and operator dependent. Consequently, additional methods for dealing with overlay error are desired.